A configuration of a solid-state imaging device is known, in which a photoelectric conversion unit and a peripheral circuit unit are allocated to and formed on separate substrates, respectively, and connected electrically to each other by microbumps or the like.
Japanese Patent Application Laid-Open No. 2009-170448 discusses a back-side surface irradiation type solid-state imaging device configured such that a first semiconductor on which pixels each including a photoelectric conversion unit and a reading circuit for reading signals therefrom are arranged, is bonded to a second semiconductor on which a signal processing for processing signals read from the pixels is arranged. The reading circuit includes transistors such as a transferring transistor, an amplifying transistor, and a resetting transistor.
In the solid-state imaging device discussed in Japanese Patent Application Laid-Open No. 2009-170448, at least the resetting transistor is arranged on the first semiconductor substrate on which the photoelectric conversion unit is arranged. An element isolation structure according to element isolation due to insulating film arranged in the semiconductor substrate, such as local oxidation of silicon (LOCOS) isolation, or shallow trench isolation (STI), is necessary for element isolation of the resetting transistor or the amplifying transistor from the photoelectric conversion unit. Dark current is liable to be generated at an interface between the insulating film arranged in a semiconductor substrate and the semiconductor substrate.
Accordingly, when the element isolation structure due to the insulating film arranged in the semiconductor substrate is used for element isolation of the photoelectric conversion unit, dark current easily flows into the photoelectric conversion unit. The dark current flowing into the photoelectric conversion unit causes noise. Noise generated at a preceding stage at which signals are amplified by the amplifying transistor dominantly affects image quality, as compared with noise generated at a subsequent stage of the amplifying transistor.
When the insulating film arranged in the semiconductor substrate is not used for element isolation of the resetting transistor and the photoelectric conversion unit, a wider element isolation region is needed to electrically isolate the resetting transistor and the photoelectric conversion unit from each other. Thus, if the light receiving area of the photoelectric conversion unit is set to be constant in order to maintain sensitivity of the photoelectric conversion unit, a pitch of the pixels increases. Consequently, it is difficult to miniaturize the pixels.